System of this kind for direct injection of fuel into the combustion chamber of an internal combustion engine are generally known. A first operational mode (a so-called stratified layer operation) and a second operating mode (a so-called homogenous operation) are distinguished from each other. The stratified layer operation is especially used at low loads; whereas, the homogeneous operation is applied for larger loads applied to the engine. In the stratified layer operation, the fuel is injected during the compression phase of the engine into the combustion chamber and to the direct vicinity of a spark plug. This has the consequence that no uniform distribution of the fuel in the combustion chamber can take place. The advantage of the stratified layer operation is that the applied lower loads of the engine can be carried out with a very low mass of fuel. Larger loads can, however, not be satisfied by the stratified layer operation. In the homogeneous operation, which is provided for such larger loads, the fuel is injected during the induction phase of the engine so that a turbulence and therefore a distribution of the fuel in the combustion chamber can easily take place. Insofar, the homogeneous operation approximates the operational mode of the engine wherein fuel is injected into the intake manifold in a conventional manner.
In both modes of operation (that is, in the stratified layer operation and in the homogeneous operation), the fuel mass to be injected is controlled (open loop and/or closed loop) by a control apparatus in dependence upon a plurality of parameters to an optimal value with a view of saving fuel, reducing exhaust gas and the like. This control (open loop and/or closed loop) is then, inter alia, dependent upon a desired value which is computed by the control apparatus. The desired torque defines the torque, which in total is to be outputted by the engine, that is, that torque which the engine should generate.
Especially during idle of the engine, it is desired that the rpm of the engine is as constant as possible. For this purpose, a method is known from U.S. Pat. No. 4,344,397 with which the idle of the engine can be stabilized. In this method, the rpm of the engine is influenced via a shifting of the ignition time point and therefore is held approximately constant. The known method is, however, only usable in an internal combustion engine wherein fuel is injected into the intake manifold in the conventional manner.
For an internal combustion engine of the type initially mentioned herein (especially in the first operating mode, that is, in stratified layer operation), the actual torque, which is generated by the engine, and therefore the rpm of the engine, is essentially only dependent upon the injected fuel mass. A change of the ignition time point is not possible in the same manner as for homogeneous operation because the igniting condition would change thereby. For this reason, the known method cannot be purposefully used for the internal combustion engine of the type initially mentioned herein.